1. Field of the Invention
The present invention relates to an apparatus for detecting over current in a plasma display panel, and more particularly, to an apparatus for detecting lowering of at least first and second output voltages from a driving apparatus of a plasma display panel below a corresponding regular voltage due to over current in the plasma display panel and generating an output control signal to be transmitted to the driving apparatus.
2. Description of the Related Art
FIG. 1 shows an apparatus for detecting over current in a conventional plasma display panel. Referring to the drawing, a first output voltage Vo1 input to a first input port 11, the level of which is dropped by resistors R101, R102, R103 and R104, is input to a negative input port (xe2x88x92) of a first comparator Co1. The first comparator Co1 generates an output control signal in a high state indicating that there is an over current flow in a plasma display panel (not shown) when the input first output voltage Vo1 is lower than a first reference voltage Vref1. Also, the first comparator Co1 generates an output control signal in a low state indicating that there is a normal current flow in the plasma display panel when the input first output voltage Vo1 is higher than or equal to the first reference voltage Vref1. The output control signal generated from the first comparator Co1 is input to a driving apparatus (not shown) of the plasma display panel via a diode D11 and a control output port SC. Devices D13, R109, ZD11, ZD12 and ZD13 connected in series between the first input port 11 and the control output port SC protect a circuit by allowing a first output voltage Vo1 at an upper limit level only to be input when the first output voltage Vo1 becomes abnormally high. Reference numeral G denotes a ground port.
Likewise, a second output voltage Vo2 input to a second input port 12, the level of which is dropped by resistors R105, R105, R107 and R108, is input to a negative input port (xe2x88x92) of a second comparator Co2. The second comparator Co2 generates an output control signal in a high state indicating that over current flows in the plasma display panel when the input second output voltage Vo2 is lower than a second reference voltage Vref2. Also, the second comparator Co2 generates an output control signal in a low state indicating that there is a normal current flow in the plasma display panel when the input second output voltage Vo2 is higher than or equal to the second reference voltage Vref2. The output control signal generated from the second comparator Co2 is input to the driving apparatus of the plasma display panel via a diode D12 and the control output port SC. Devices D14, D15, ZD15, ZD16, ZD17, ZD18, R110 and D16 connected in series between the second input port 12 and the control output port SC protect the circuit by allowing a second output voltage Vo2 at an upper limit level only to be input when the second output voltage Vo2 becomes abnormally high.
The conventional over current detecting apparatus has the following problems as it uses the comparators with respect to each of the output voltages Vo1 and Vo2.
First, as transmission delay time of each of the comparators Co1 and Co2 is relatively long, the over current flowing in the plasma display panel cannot be detected rapidly.
Second, as each of the output voltages Vo1 and Vo2 is compared separately, it is not suitable for a plasma display panel having various levels of driving voltages.
Third, as the reference voltages Vref1 and Vref2 of the comparators Co1 and Co2 must be accurately generated, the configuration of a circuit of the apparatus becomes relatively complicated.
To solve the above problems, it is an objective of the present invention to provide an apparatus for rapidly detecting over current flowing in a plasma display panel with simple circuitry even when there are various levels of the driving voltage in the plasma display panel.
Accordingly, to achieve the above objective, there is provided an apparatus for detecting lowering of at least first and second output voltages from a driving apparatus of a plasma display panel than a corresponding regular voltage due to over current in the plasma display panel and generating an output control signal to be transmitted to the driving apparatus, which comprises a switching transistor having a collector to which a bias voltage of a positive polarity is applied, an emitter which is grounded, and a base, the switching transistor being operated according to the level of a voltage applied to the base and generating the output control signal through the collector, a voltage signal generating portion connected between the base of the switching transistor and a port of the first output voltage, and the base and a port of the second output voltage, respectively, for changing a bias voltage of the base as the first and second output voltages change, and a switching support portion connected between the emitter of the switching transistor and at least any one of the first and second output ports, for allowing a lowered voltage to be applied to the emitter when at least any one of the first and second output voltages is lower than a corresponding regular voltage, to thus prevent incorrect operation of the switching transistor.
The effects of the present invention are as follows.
First, as a switching transistor is used, the transmission delay time is relatively short so that over current flowing in the plasma display panel can be rapidly detected.
Second, as the output voltages for driving the plasma display panel are integrally monitored by the switching transistor, the over current detecting apparatus is suitable for a plasma display panel having various levels of a driving voltage.
Third, a reference voltage for comparison need not be generated as the switching transistor is used. Thus, the configuration of the apparatus becomes relatively simple.
Fourth, as incorrect operation of the switching transistor is prevented by the switching support portion, detecting over current can be performed more accurately.